Photothermographic imaging materials that are classified as "dry silver" compositions or emulsions comprise a light-insensitive reducible silver source; a light-sensitive silver source; and a reducing agent for the light-insensitive, reducible silver source. The light-sensitive material is generally photographic silver halide, which must be in catalytic proximity to the light-insensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source by the reducing agent. It has been long understood that silver halide is a catalyst for the reduction of silver ions and that the silver-generating, light-sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as by partial metathesis of the reducible silver source with a halogen-containing source (e.g., see U.S. Pat. No. 3,457,075); coprecipitation of silver halide and reducible silver source material; (e.g., see U.S. Pat. No. 3,839,049); and other methods that intimately associate the silver halide and the silver source.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms. The imagewise distribution of these clusters is known in the art as a latent image. As this latent image generally is not visible by ordinary means, the light-sensitive article must be further processed in order to produce a visual image. The visual image is produced by the catalytic reduction of silver ions which are in catalytic proximity to the silver halide grains bearing the latent image.
One conventional way of attempting to increase the image density of photographic and photothermographic emulsions without increasing, or while decreasing, the amount of silver in the emulsion layer is by the inclusion of dye forming materials into the emulsion. In this way a dye enhanced silver image can be produced.
Residual silver stain is a major problem with dry silver color constructions known in the art. This has been overcome by causing the developed dye image to diffuse from the dry silver layer to an image-receiving layer that is then stripped from the dry silver layer.
U.S. Pat. No. 4,594,307 discloses a heat developable photographic material that produces a pure and stable dye image by the oxidation-reduction reaction between a reducible organic silver salt and a leuco dye reducing agent wherein the dye formed is transferred to an image-receiving layer by continuing the heating for development. This separates the dye formed from the silver images and other residual chemicals.
It has been described in the patent literature to transfer a dye image formed in a photothermographic system by means of a transfer solvent; see, for example, U.S. Pat. Nos. 3,985,565; 4,021,240; and 4,022,617.
Japanese Patent Application No. 59-5239 discloses a photothermographic contact diffusion system wherein a chemical reaction occurs in an image-receiving layer between a diffused leuco dye and an acidic color developing agent.
Heat developable photographic materials for providing dye images by the reaction of color couplers with the oxidants of an organic reducing agent have been described in U.S. Pat. Nos. 3,531,286; 3,761,270; and 3,764,328. These materials suffer from the problem that the optical density of the background is increased because of the presence of unreduced silver. Poor print stability is also a problem.
Dye formation by an oxidation-reduction reaction between a reducible silver source and a leuco dye to form a visible dye is disclosed in U.S. Pat. Nos. 3,985,565; 4,022,617; and 4,460,681. However, in these processes, the materials provide turbid and hazy color images on account of the presence of the reduced silver image after heat development. Moreover, the image tends to suffer from background stain upon aging due to residual chemicals in the material. The silver images can be removed by liquid processing and the dyes can be transferred to an image-receiving layer with the aid of a transfer solvent such as alcohol.
Another process employing a heat developable photographic material to produce dye images by the oxidation-reduction reaction between an organic silver salt oxidizing agent and a dye releasing compound that releases a mobile dye when the material is heated is disclosed in Japanese Patent Application Nos. 58-58543; 58-79247; 58-149046; and 58-149047. This process requires that the dyes be transferred to an image-receiving sheet with the aid of a transfer solvent such as water.
Depending on the particular ingredients of a given dry silver layer, the development may be best carried out, for example, under acidic or basic conditions. When multiple dry silver layers with incompatible developing chemistries are employed, it is very difficult to keep development conditions within the dry silver layer from affecting the development of nearby or adjacent dry silver layers. As a result, it is advantageous to coat dry silver layers with different developing conditions on opposite sides of a transparent substrate. However under these conditions a good reflection print has not been obtained.
Opacifying layers have been employed in the wet-developed photographic art to improve the quality of reflection prints. For example, U.S. Pat. No. 3,928,037 and references cited therein describe the use of opacifying layers in diffusion transfer reversal photographic materials. Those materials are not exposed through the opacifying layer and are wet diffusion processed.